Applications can use information about the language of content to deliver to users the most appropriate information, or to present information to users in the most appropriate way. The more content is tagged and tagged correctly, the more useful and pervasive such applications will become.

Language information is useful for accessibility, authoring tools, translation tools, font selection, page rendering, search, and scripting.

Information about the language of a document is extremely important for screen readers and accessibility, right from the outset. These applications need to know whether they can produce output from the text, or whether perhaps they need to switch to a different language mode.

Authoring tools can use language information for such things as spelling and grammar checking. To achieve, for Web content authoring, the kind of support provided in products such as Microsoft Office it is essential that authors know how to associate their documents and text with language information and do so.

Some browsers use language information to determine appropriate fonts for Simplified vs. Traditional Chinese vs. Japanese vs. Korean. Although, on a page encoded in Unicode, these languages may share the same code points for ideographic characters, there is an expectation on the part of speakers of these languages that the glyphs used should vary in small details. The illustration on the slide shows the affect on text of changing nothing but the language tag in a Mozilla browser. (You can try this out for yourself using the test page it is taken from.)

Marking up language information also aids in applying appropriate stylistic variations. For example, fonts or line spacing may need to change to accommodate different alphabets, style-generated quotation marks may need to be different by language, emphasis may need to be expressed in language dependent ways, etc.

Marking up language information also allows for language-specific processing. For example, an XSLT process could be used to extract text ordered in the appropriate way for the language of the document. Alternatively, using the XSLT lang() function it is possible to extract language-specific text from a file. As another example, you could use language information to apply culture-specific styling, such as appropriate quote substitution or emphasis, during conversion to XSL-FO.